1982 w123, US

After much tribulation, I was able to get the tumbler to turn to the first position to get the tumbler out. Then with the tumbler out, I was able to get the lock assembly out of the car and on the bench now. I'm relieved that I don't have to drill this out of the car. But, I'm afraid to reinstall it and have the same problem. With the lock assembly on the bench and the tumbler out of it, shouldn't I easily be able to turn the lock with a screw driver to any position? It still won't turn with a screw driver. To me, this means that the tumbler isn't the problem. What is likely the problem now? BTW, this lock assembly is only a few years old.